The present invention relates to an air flow rate measuring device, and in particular to an air flow rate measuring device for measuring an air flow which is supplied to an internal combustion engine of an automobile.
Generally, a metal mesh, an aluminum honeycomb or a plastic is used to form a rectifier grid of an air flow rate measuring device. In the case of a plastic molding, as described in Japanese Patent No. 3245362, in order to maintain a mechanical strength and prevent deformation and deterioration of the grid, the plastic molding is configured to include a first mesh grid having a first thickness and spaces therein and a second rib grid having a thickness larger than the first one and spaces wider than in the first mesh grid, and the first mesh grid and the second rib grid are integrally formed with one another. Also, in order to address a load applied toward a center of a body of a device from an outer periphery thereof, a grid configuration is known which has a hexagonal second rib grid such as a honeycomb and a first mesh grid between the second rib grid so that the applied radial load can be spread out.
In the prior art, a deformation of the second rib grid leads to a deformation of the first mesh grid, and also a deformation of the second rib grid at the beginning of a tightening of a hose band or due to the radial load caused by a thermal factor causes a problem of deterioration of measurement accuracy of the air flow rate measuring device. In addition, the second rib grid is formed to have a sufficient thickness to maintain a mechanical strength of the rib grid, which results in a large pressure loss across the grid.
Therefore, there is a need for providing an air flow rate measuring device in which deterioration of its measurement accuracy can be prevented and a reduction of a pressure loss across a grid can be achieved in association with the radial load which is generated by a tightening of a hose band.
A tightening of a hose band generates two types of radial loads: one type is the load applied due to a configuration factor at the beginning of a tightening of a hose band, and another type is the load caused by a thermal factor, that is difference in coefficient of linear thermal expansion between the hose band and the body and a rectifier grid due to temperature changes. Therefore, the radial loads due to the two factors generate a stress which causes a deformation of a grid, and the deformation should be taken into the consideration to address the problem which is caused by the radial load at tightening the hose band.
In addition, because a resin exhibits creep, a deformation of a grid caused by the creep also should be taken into the consideration to address the problem.
One object of the present invention is to provide a flow measuring device or a flow measuring passage having grids which are not easily deformed.